This invention relates to two-way communications systems and more particularly to an improved system for permitting remote use of a mobile transceiver.
In U.S. Pat. No. 3,745,462, the disclosure of which is hereby incorporated by reference, a mobile bidirectional communication system is disclosed in which a plurality of conventional mobile transceivers, typically mounted in an automotive vehicle, are each provided with a mobile radio extension unit permitting the receiver and transmitter of the mobile transceiver to function as a repeater or as a transponder in connection with a handheld portable transceiver of limited range located in the proximity of the automotive vehicle. Each mobile radio extension unit is provided with a separate transmitter and receiver having a different operating frequency f.sub.2 from that of the mobile transceiver f.sub.1 for enabling simplex mode bidirectional communication between any portable transceiver within range and the base station or a different mobile transceiver from that associated to a selected mobile radio extension unit. The system operates in such a manner that only one mobile radio extension unit in the network can communicate between one or more portable transceivers and the base or other station at any given time, the remaining extension units in the network being disabled by contemporaneous detection of the carrier frequency f.sub.2 used to communicate between the selected extension unit and the portable transceiver, and the carrier frequency f.sub.1 used to communicate between the selected extension unit and the base or other station. In order to provide automatic selection of a single extension unit, each unit is provided with an internal sampling clock having a different sampling frequency from the other units for enabling periodic detection of the base station carrier f.sub.1, the extension unit carrier f.sub.2 and a signalling tone generated by the portable transceiver when the operator desires to transmit a message to the base or other station via an extension unit. Since the sampling clocks have different frequencies, there is a high statistical probability that no two clocks will simutaneously enable their associated units, and each unit is provided with control circuitry for preventing active operation when both the base station carrier f.sub.1 and the mobile radio extension unit carrier f.sub.2 are being received.
The above system can be applied to a wide range of applications with beneficial results. In one application, the base station comprises a police or fire department dispatcher while the mobile transceivers comprise conventional units presently used in police or fire vehicles. The mobile radio extension units are also located in the vehicles and connected to the associated mobile transceiver. With this configuration, none of the normal functions of the mobile transceivers are impaired, but the portable transceivers can be employed by a policeman or fireman when away from the vehicle. Since each portable transceiver need only have the capacity to communicate with a nearby vehicle-mounted MRE Unit, the portable transceivers are extremely small in size and lightweight. In addition, since any portable transceiver is capable of communicating with any MRE unit, it is not necessary to assign specific portable transceivers to specific MRE units.
However, in the above system, once the mobile radio extension unit is engaged in the repeat to portable mode. the portable transceiver is incapable of transmitting a message back to the base station or to another mobile transceiver until the signal being received on the base station frequency f.sub.1 terminates. In many circumstances, this inability of the portable transceiver to interrupt the reception of a signal from the base station or another mobile transceiver and transmit a message back to the base station or the other mobile transceiver, a process which is commonly known as "punch through", is undesirable. For example, if a policeman on foot patrol encounters a sudden emergency when a message is being received on his portable transceiver, the urgency of the emergency may require that a message be immediately transmitted back to the base station or other mobile transceivers in the network (such as a request for assistance, an ambulance, or the like), rather than after an indeterminate period of transmission to the portable transceivers in the network. Similarly, under certain atmospheric conditions skip signals from an extra-network transmitter can be received by the mobile transceivers in the network and repeated to the portable transceivers. Without the ability to punch through these interfering signals, communication from the portable transceivers to the base station or other mobile transceivers in the network is blocked. To date, efforts to design a communications network having the advantages of the system described above and the added advantage of a punch through capability have not met with success.